The present invention relates to a color image forming apparatus used in color printers, color copying machines or color facsimiles. More specifically, the present invention relates to a color image forming apparatus that forms a color toner image by overlapping several toner images of various colors on an intermediate transfer device in a primary transfer from several photosensitive drums, and transcribing the color toner image in a secondary transfer to a transfer material (recording paper).
FIG. 11 shows the internal structure of a prior art example of a color image forming apparatus, as disclosed in Publication of Unexamined Patent Application (Tokkai) No. Hei 7-36246.
The printer comprises an intermediate transfer belt unit 101 including an-intermediate transfer belt 102, a primary transfer roller 103, a secondary transfer roller 104, a cleaner roller 105, and a waste toner reservoir 106. Composition or superposition of color toner images is performed on the transfer belt 102. A group of image forming units 108 is made up of four image forming units 107Bk, 107Y, 107M and 107C, each unit being of sector shape in cross section. As can be seen in FIG. 11, the image forming units are arranged circularly in the middle of the printer.
When an image forming unit 107Bk, 107Y, 107M or 107C is set properly in the printer, mechanical and electrical connection systems are established between one of the image forming units 107Bk, 107Y, 107M and 107C and the machine body side via mutual coupling members. The image forming units 107Bk, 107Y, 107M and 107C are supported by a supporter, which is rotationally driven by a motor via a cylindrical shaft 109. Each image forming unit 107Bk, 107Y, 107M, and 107C is successively moved by rotation to an image forming position 110. The image forming position 110 is the position where a photosensitive drum 118 of the image forming unit faces the intermediate transfer belt 102 on the primary transfer roller 103, and is also the exposure position for exposure by a laser beam 111.
A laser exposing device 112 is provided in the lower part of the printer. The laser signal beam 111 from the laser exposing device 112 w passes through an opening 113 between the image forming units 107M and 107C, and through an opening provided in the cylindrical shaft 109, and enters a mirror 114. This mirror 114 is positioned inside the shaft 109 and fixed directly to the machine body. The reflected laser beam 111 enters the image forming unit 107Bk located at the image forming position 110 through an opening 115, and passes through the space between a developing device 116 and a cleaner 117 of the image forming unit 107Bk, and enters an exposure portion of the photosensitive drum 118. The laser signal beam 111 is scanned by the exposing device in the direction of the axis of the photosensitive drum 118. The toner image, which is formed on the photosensitive drum 118 by exposure with the laser signal beam 111 and subsequent development with the developing device 116, is transferred to the intermediate transfer belt 102.
Then, the group of image forming units 108 rotates by 90 degrees, so that the yellow image forming unit 107Y moves to the image forming position 110 to replace the black image forming unit 107Bk. An operation similar to the operation explained above for the black image is performed to form a yellow image overlaying the black image formed on the intermediate transfer belt 102. Subsequently, the magenta and cyan image forming units 107M, 107C are moved to the image forming position 110, and similar operations as explained above are repeated to compose a full color image on the intermediate transfer belt 102. This full color image is further transferred from the intermediate transfer belt 102 onto a recording paper using a secondary transfer roller 119, and the image on the paper is fixed by a fixing device 120.
In the color image forming apparatus as explained above, precise registration of the toner images of all four colors is very important for obtaining a high quality full color image. However, an image forming unit system of the prior art as explained above, which forms a color image by overlaying successively four toner images of four photosensitive drums at one image forming position onto an intermediate transfer device (belt) and forms a color image on the recording paper by a secondary transfer from the intermediate transfer device, has the following disadvantage: After the start-up (power on), variations of the time until a rotation of the intermediate transfer device is in a stable condition (start-up time) and loss of the driving system can occur easily. Therefore, the rotational position of the intermediate transfer device at a predetermined time after the start-up is not necessarily a predetermined position. Consequently, it is necessary to ensure that sufficient time has passed so that the rotation of the intermediate transfer device is in a stable condition in order for a high quality image to be provided.
On the other hand, there is the strong desire to accelerate the recording operation. The time spent until the four image forming units comprising a photosensitive drum have been switched, and the rotation of the intermediate transfer device has been stabilized, is reflected in the time that the color image forming device needs for forming an image.
Moreover, the circumference of the intermediate transfer belt has an influence on the size of the entire device. Therefore, to make the circumference of the intermediate transfer belt as small as possible is desirable in order to miniaturize the device.
A main object of the present invention is to solve the problems mentioned above by providing a color image forming apparatus combining improvement of the image quality and the image forming speed with miniaturization of the entire device.
A color image forming apparatus according to the present invention comprises: a plurality of image forming units corresponding to various colors, the image forming units including a photosensitive drum, a charging device and a developing device; a unit retaining member for retaining the image forming units and moving the image forming units between an image forming position and a waiting position; an exposing device for exposing the photosensitive drum of the image forming units when located at the image forming position; an intermediate transfer belt for successive transfer and superposition of toner images of various colors from the photosensitive drum of the image forming units located at the image forming position so as to form a color toner image; means for driving the photosensitive drum and the intermediate transfer belt; a detector for detecting a reference position of the intermediate transfer belt when the intermediate transfer belt is driven, and outputting a reference position detection signal; a secondary transfer device for transferring the color toner image on the intermediate transfer belt onto paper; and a controller for controlling the operation of the above structural elements. The controller determines an operation start time of the charging device and the developing device using a rotation command signal to the driving means as a reference, and an operation start time of the exposing device, the intermediate transfer belt and the secondary transfer device using the reference position detection signal as a reference.
Usually, the start and the stop of the photosensitive drum and the intermediate transfer belt is performed for each color. In that case however, variations of the time until a rotation of the intermediate transfer device is in a stable condition (start-up time) and loss of the driving system can occur easily. In the configuration according to the present invention however, the detector detects a reference position of the intermediate transfer belt after the driving of the intermediate transfer belt is started and outputs this reference position detection signal. The positioning of the overlayed toner image, which is transferred onto the intermediate transfer belt, becomes easier and more precise, because the operation start time for the exposing device, the intermediate transfer belt and the secondary transfer device are determined using the reference position detection signal. Positioning for the secondary transfer from the intermediate transfer belt onto paper becomes more precise as well.
On the other hand, the charging of the photosensitive drum before the photosensitive drum reaches a constant rotational velocity can be performed without problems. It is preferable that the charging of the photosensitive drum starts as early as possible after the start of the photosensitive drum, so that a larger charging portion on the surface of the photosensitive drum can be ensured. Especially in the case of the contact development method, wherein the developing roller is always contacting the photosensitive drum, it is preferable that a developing bias is impressed as early as possible after the start of the photosensitive drum for development. If this is not done, an unwanted use of toner can occur, because toner covers an unexposed area of the photosensitive drum as well. According to the configuration of the present invention, the charging and the developing of the photosensitive drum can be started as early as possible after the start of the photosensitive drum, because the operation start time of the charging device and the developing device are determined using a rotation command signal to the driving means as a reference.
It is preferable that a selection between a first control mode and a second control mode is possible, the first control mode being characterized in that the exposure by the exposing device begins after the photosensitive drum charged by the charging device has been rotated for at least one rotation, and the second control mode being characterized in that the exposure by the exposing device begins before the photosensitive drum charged by the charging device has been rotated for one rotation. The first control mode is a high image quality mode, wherein the exposure is started when the electric potential of the photosensitive drum is in a sufficiently stable condition, and the second control mode is a high speed mode, wherein a high recording speed is preferred. It is furthermore preferable that, when the first control mode is selected, the controller starts the rotation of the intermediate transfer belt after the photosensitive drum has been rotated for at least one rotation. By doing so, the length of the intermediate transfer belt can be shortened, and thus the miniaturization of the entire device can be enhanced.
It is preferable that (i) the secondary transfer device comprises a secondary transfer roller that can be switched between a state in contact to the intermediate transfer belt and a state in separation from the intermediate transfer belt, (ii) the color toner image is transferred from the intermediate transfer belt onto paper while the paper passes by when the intermediate transfer belt and the secondary transfer roller are in contact, and (iii) in the case that the first control mode has been selected, the controller maintains a separation between the secondary transfer roller and the intermediate transfer belt while the photosensitive drum is being exposed by the exposing device and while the toner image is being transferred from the photosensitive drum to the intermediate transfer belt. By doing so, the running of the intermediate transfer belt can be stabilized and an image with a high image quality can be formed.
It is preferable that the color image forming apparatus further comprises a paper feed device for feeding paper synchronized by the reference position detection signal. The paper feed device is controlled by the controller so that, in the case that the first control mode has been selected, the paper feed device does not feed paper while the photosensitive drum is being exposed by the exposing device and while the toner image is being transferred from the photosensitive drum to the intermediate transfer belt. By doing so, disturbances due to the paper feed operation can be prevented and an image with a high image quality can be formed.
It is preferable that the color image forming apparatus further comprises a cleaning means, which can be switched between a state in contact to the intermediate transfer belt and a state in separation from the intermediate transfer belt, and cleans remaining toner from the surface of the intermediate transfer belt in the contact state. In the case that the first control mode has been selected, the controller maintains a separation between the cleaning means and the intermediate transfer belt while the photosensitive drum is being exposed by the exposing device, while the toner image is being transferred from the photosensitive drum to the intermediate transfer belt and while the toner image is being transferred by the secondary transfer device from the intermediate transfer belt onto paper. By doing so, the running of the intermediate transfer belt can be stabilized and an image with a high image quality can be formed.
It is preferable that the controller stops the driving means while the plurality of image forming units retained by the unit retaining member is moved, and the image forming unit corresponding to a color of the plurality of image forming units that is not being used for image formation is skipped and only the image forming units corresponding to colors that are used for image formation are moved successively to the image forming position. By doing so, the different colors of the color toner image can be overlayed on the surface of the intermediate transfer belt without position variation while the color image formation can be accelerated.
It is preferable that the driving means has a single driving source driving the photosensitive drum and the intermediate transfer belt, which driving source, after being stopped by the controller, can be driven in reverse to run back the intermediate transfer belt for a predetermined length when the image forming units are switched. With such a driving method, a miniaturization of the entire device can be enhanced, because the length of the intermediate transfer belt can be shortened. It is even more preferable that the color image forming apparatus further comprises a driving force interrupting means for interrupting the transmission of a driving force from the driving means to the photosensitive drum, wherein the controller interrupts the transmission of a driving force with the driving force interrupting means when the driving source is driven in reverse to run back the intermediate transfer belt for a predetermined length.
It is preferable that the controller can drive the driving source in reverse to run back the intermediate transfer belt for a predetermined length while the image forming units are moving. It is even more preferable that the running direction of the intermediate transfer belt in a portion facing the photosensitive drum is the same direction as the moving direction of the image forming unit when the driving source has been driven back. In this case, the length of the intermediate transfer belt can be shortened and the life expectancy of the intermediate transfer belt can be prolonged, because friction between the photosensitive drum and the intermediate transfer belt can be kept low when the image forming unit is moved.